Chemical vapor deposition methods are employed to form films of material on substrates such as wafers or other surfaces during the manufacture or processing of semiconductors. In chemical vapor deposition, a chemical vapor deposition precursor, also known as a chemical vapor deposition organometallic compound, is decomposed thermally, chemically, photochemically or by plasma activation, to form a thin film having a desired composition. Typically, a chemical vapor deposition organometallic precursor can be contacted with a substrate that is heated to a temperature higher than the decomposition temperature of the precursor, to form a metal or metal oxide film on the substrate.
For example, aluminum interconnects are used in the manufacture of DRAM type microchips and flash memory devices. Aluminum is a good conductor of electricity and is easy to deposit by chemical vapor deposition. Aluminum oxide (alumina) is an insulator used in microchip manufacture. A popular precursor for aluminum and alumina chemical vapor deposition is trimethylaluminum. Trimethylaluminum, however, is extremely pyrophoric and difficult to handle. 1-Methylpyrrolidinealane (MPA) is a free flowing, non-pyrophoric precursor to aluminum and alumina with good stability, a low decomposition temperature and a high vapor pressure.
The synthetic processes utilized to generate organometallic precursors are highly important, and must insure safety, high purity, throughput, and consistency. However, the air-sensitive nature of the starting materials make the synthesis of organometallic precursors more challenging. When starting materials are solids, it makes air-free transfer more difficult. Employing reactants that cannot be solvated can cause mixing and handling problems in the process, especially in large scale production. Furthermore, the use of reactants that cannot be solvated can lead to lower product yields. Therefore, developing a methodology for producing organometallic precursors that addresses the aforementioned potential hold-ups would be beneficial toward establishing the production of these materials for use in the electronics industry.
Current manufacturing methods for 1-methylpyrrolidinealane involve the use of various solvents. For example, U.S. Pat. No. 6,143,357 discloses a process for making organometallic compounds, e.g., 1-methylpyrrolidinealane, by forming a suspension of aluminum trichloride and lithium aluminum hydride in hexane or pentane and adding a Lewis base, e.g., 1-methylpyrrolidine, to the suspension. Hexane is an OSHA toxic chemical with a PEL of 500 ppm. A spill of one pound of n-hexane requires notification of the National Response Center. Both lithium aluminum hydride and aluminum trichloride are solids and neither is soluble in pentane or hexane. This leads to difficulties in large scale production (handling of air sensitive solids) and lower yields.
Marlett and Park (Marlett, E. M.; Park, W. S. J. Org. Chem., 1990, 55, 2968) reported the synthesis of 1-methylpyrrolidinealane by the action of N-methylpyrrolidine on lithium aluminum hydride in toluene. A reactive solid (trilithium aluminum hexahydride) was produced as a byproduct.
Frigo and van Eijden (Frigo, D. M.; van Eijden, G. J. M. Chem. Mater., 1994, 6, 190) reported a process for making amine adducts of alane by adding a stoichiometric amount of an amine to a slurry of lithium aluminum hydride and aluminum chloride in the presence of an alkane solvent, e.g., pentane. Frigo and van Eijden state that the use of ether solvents “is very likely to give contamination of the end product by the ether, which could lead to incorporation of oxygen in the Al-containing layers.”
Amine adducts of alane have been used in organic synthesis for the reduction of organic compounds to alcohols and amines. As such, there are several published methods of making alane amines. A common method is the reaction of lithium aluminum hydride with trialkylammonium chlorides as reported by Ruff and Hawthorne (Ruff, J. K.; Hawthorne, M. F. J. Amer. Chem. Soc., 1960, 82, 2141) using ethereal solvents or Kovar and Callaway (Kovar, R. A.: Callaway, J. O. Inorg. Synth., 1977, 17, 36) using aromatic solvents. Ruff and Hawthorne employ solid reagent addition in their experimental methods. Also, the hydrochloric acid salt of N-methylpyrrolidine is insoluble in ethereal solvents.
The prior art processes used to produce 1-methylpyrrolidinealane have various disadvantages as discussed above. Therefore, a need continues to exist for new processes for making organometallic precursors that are safer (e.g., no hazardous solvents), allow for easier handling of reactants (e.g., allow the solid reagents to be dissolved and transferred in solution through the use of metering pumps), give higher product yields and permit easier scale up for production quantities of organometallic compounds. It would therefore be desirable in the art to provide new processes for making organometallic compounds that address these needs.